High-Homogeniety Solenoids

The magnetic field in a simple solenoid decreases approximately quadratically with the axial distance from the center of the magnet. The homogeneity at the center of the magnet can be improved significantly by adding windings to each end of the solenoid to compensate for this decrease in the field. Mathematically, this technique results in a very uniform magnetic field at the center of the coil. In practice, however , winding imperfections limit the homogeneity attained and additional series connected trim windings are required to achieve the maximum homogeneity. Since the trim coils are connected in series, this type of magnet has only two current leads and one persistent switch is used for the complete magnet. The homogeneity of such a magnet is about ± part in 10 5 of the central field in a spherical volume having a diameter that is approximately 1/5 of the bore of the magnet in small magnets.

The limit on this technique is the change in field homogeneity caused by adding or removing one turn of conductor from the magnet. A typical magnet of this type might have 3 x 10 4 turns, so each turn contributes 3 x 10 -5 of the total field, to a first approximation. Field homogeneity of 1 part in 10 6 require something other than adding or deleting turns. The procedure used is to add separately energized trim coils to the magnet so the field contribution of each coil can be varied. Each of these coils is fitted with a separate persistent switch. By the nature of this correction, each of these coils operate at currents lower than the main magnet. Magnets with these separate trim coils have a homogeneity of about 1 part in 10 6 of the central field. The ratio of the magnet bore diameter to the diameter of the homogeneous volume typically increases as the specified homogeneity increases.

Extending this procedure to the next set of trim coils results in coils operating at still lower currents. These currents are small enough that copper wires can be used and the coils can be operated at room temperature. Homogeneities of about 1 part in 10 7 of the central field can be achieved by the addition of these coils.

A great amount of emphasis has been placed on extremely homogeneous and persistent magnets in recent years. These magnets obviously fulfill a need in many experiments. Not all NMR experiments require these features, however, and money can be saved by specifying only the parameters that are required for your particular experiment.

Magnets of this type are used extensively for NMR experiments as well as for some precision transport phenomena measurements.

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